More than 50 world leaders come to Chicago this weekend for a summit meeting of NATO, the North Atlantic Alliance. They will discuss, among other things, how to end the war in Afghanistan. They will talk in a city where anti-war protestors famously clashed with police outside the 1968 Democratic convention. In 2012, anti-war activists are making preparations. NPR's David Schaper reports.

DAVID SCHAPER, BYLINE: Though most of the leaders, diplomats and generals from the 28 NATO nations have yet to arrive in Chicago, thousands of protesters have, and are already demonstrating against the NATO summit.

SCHAPER: This protest included the groups CODEPINK: Women for Peace, The World Can't Wait, Veterans for Peace and others. And there have been similar rallies and marches every day in Chicago this week, all of them peaceful, though 12 protesters have been arrested for acts of civil disobedience. DePaul University Professor Tom Mockaitis.

TOM MOCKAITIS: You have dozens of groups showing up here protesting everything from the summit itself to using this as an opportunity to showcase messages on virtually every cause that you can imagine associated with anger at government, at the economy, at national defense policy, and so on.

SCHAPER: Many of the groups had been planning on targeting the G8 summit, more so than NATO. But when President Obama moved that meeting to Camp David, those activists focused on the economy, kept their plans to come to Chicago and shifted their focus to NATO.

Most of the demonstrations are expected to be peaceful, but authorities are preparing for the possibility of violence, as some anarchists have been promoting violent confrontations in Chicago online. Mockaitis says in Seattle, London, Toronto and other cities, anarchists have been using what's known as the black bloc technique: They blend into an otherwise peaceful demonstration, but are dressed in all black, and at some point, they break out and form a bloc to create havoc.

MOCKAITIS: Their ability to destroy property, to provoke a strong police response, to mingle among more legitimate protesters who do follow rules is significant, and they require a great amount of personnel to counter.

SCHAPER: Mockaitis says those in the black bloc use Twitter, text messaging and other social media to organize and change plans or targets quickly. Chicago Police Superintendent Garry McCarthy says he knows that some of the protesters' goal will be to provoke the police.

GARRY MCCARTHY: We're used to being provoked.

SCHAPER: In fact, he says there was one such demonstration already this week.

MCCARTHY: Anarchists F the Police march was the name of it.

SCHAPER: McCarthy says the officers kept their cool, and there were no physical confrontations or arrests. McCarthy says policing has changed so much in the 44 years since the infamous 1968 Democratic National Convention in Chicago, that it's really not fair to compare then to now. And he says Chicago police have new tactics for dealing with large scale protests that could turn ugly.

MCCARTHY: One of them is extraction versus wholesale police line into a crowd of people. We don't want to disperse crowds. We want to take the people out of the crowd who are causing the problems.

SCHAPER: McCarthy says police would like the law-abiding protesters to help them identify the troublemakers. But Pat Hunt, one of the organizers of the anti-NATO rallies, says protesters aren't the problem.

PAT HUNT: There are some people that say the troublemakers will be meeting over here, as part of NATO.

SCHAPER: But Hunt and other planners say that they are focused on creating safe, legal and family friendly anti-NATO protest rallies this weekend.